The invention relates to an electric switching device. The electric switching device has a signaling element, e.g. a switching strip, which leads to a switching function in the event of a closed circuit being interrupted. The switching device also includes a relay safety assembly connected into the closed circuit.
Electric switching devices of this type are described e.g. in EP 234 523 B1, to be precise in connection with a closing-edge fuse. Closing-edge fuse systems of this type are provided for use on motor-driven doors and gates in buildings and public transport means, movable shelves and on protection devices for machines. A closed-circuit current flows in the actual switching strip and is monitored by an evaluating unit. Contact is made as a result of the closed-circuit current being interrupted. Load reduction is followed by automatic resetting for the purpose of closing the closed circuit.
The so-called evaluating unitxe2x80x94or also called relay safety assemblyxe2x80x94is likewise known in many different forms in the prior art and monitors itself continually by the closed-circuit principle. Faults such as short circuit-cable breakage-profile interruptions and incorrect connection lead to the disconnection of the load circuits. In the prior art, evaluating units of this type are referred to by the technical designation xe2x80x9crelay safety assemblyxe2x80x9d.
These known arrangements have the disadvantage that a single-ended earth fault is not identified since the device continues to remain functional.
The invention is based on the object of providing a line monitoring arrangement which both demonstrates a single earth fault and reacts appropriately in the event of a short circuit.
This object on which the invention is based is achieved by means of the fact that the incoming electrical line of the closed circuit to the signaling element is surrounded by a conductive safety screen which is connected to the protective conductor terminal of the relay safety assembly, the secondary circuit of the transformer in the relay safety assembly being connected to the safety screen on one side.
If such an arrangement is used in conjunction with a closing-edge fuse, a control signal which effects disconnection of the load circuits occurs even in the event of an earth fault of the incoming line of the closed circuit.
In order to illustrate the invention, a block diagram of a relay safety assembly known in the prior art, type: xe2x80x9cBN 5983xe2x80x9d, is enclosed in the Appendix, the screening according to the invention being additionally illustrated in this block diagram.